A ferroelectric (FE) material is a dielectric crystal that exhibits a spontaneous electric polarization. The direction of spontaneous polarization can be reversed between two crystallographic defined states by application of an external electric field. Ferroelectric materials exhibit ferroelectricity only below the Curie temperature Tc, which in literature is also known as the phase transition temperature. Since the phase transition temperature is a material property, its value covers a vast spectrum of temperatures. Above this temperature the material exhibits paraelectric properties and behavior.
The discovery of ferroelectric Hafnium oxide (HfO2) triggered an increase in research on ferroelectric materials, since it was widely recognized that the existence of robust, chemically stable, and relatively inert ferroelectric crystals offered an electrically switchable, two-state device. This electrically switchable two-state possibility by ferroelectric materials has been proposed to be used for different application like memory devices, negative capacity transistors (NCFET), for energy harvesting applications in electro-caloric devices, ferroelectric and pyroelectric sensors and piezoelectric devices, so-called supercapacitors.
Many electronic devices and systems have the capability to store and retrieve information in a memory structure. A number of different memory structures are used in such systems. One prominent volatile memory is a DRAM structure that allows for high speed and high capacity data storage. It is called a volatile memory structure because the stored data is lost within seconds and has to be refreshed in time to secure data storage. Some examples of non-volatile memory structures include read only memory (ROM), EEPROM (electrically erasable programmable read-only memory) and flash memories as an electronic (solid-state) non-volatile computer storage medium that can be electrically erased and reprogrammed. Ferroelectric memory structures (e.g., FeRAM and FeFET devices), phase change memory (e.g., PCRAM) structures, resistive memory (e.g., RRAM) structures and magnetic memory (e.g., MRAM) structures are examples of such non-volatile memory structures.
With regard to ferroelectric (FE) structure, these structures can be in the form of a capacitor (e.g., a FeRAM) or a transistor (FeFET), where information can be stored as a certain polarization state of the ferroelectric material within the structure. The ferroelectric material that can be used is hafnium dioxide or zirconium dioxide or a solid solution of both transition metal oxides. In the case of pure hafnium oxide, the remanent polarization can be improved by a certain amount of dopant species which has to be incorporated into the HfO2 layer during the deposition.
The ferroelectric material is intended to partially or fully replace the gate oxide of a transistor or the dielectric of a capacitor. The switching is caused by applying an electrical field via voltage between transistor gate and transistor channel. Specially, for n-channel transistors, ferroelectric switching after application of a sufficiently high positive voltage pulse causes a shift of the threshold voltage to lower or negative threshold voltage values. For p-channel transistors a negative voltage pulse causes a shift of the threshold voltage to higher or positive threshold voltage values.
One problem that can occur is that charge carriers trapping from one of the electrodes in a capacitor device can shift the polarization hysteresis by creation of an internal bias field due to unequal distribution of charges at the two different electrodes (imprint effect).
Another problem that can occur is that minority carrier trapping from the channel region can shift the threshold voltage of transistors oppositely to the direction caused by ferroelectric switching. Accordingly, it is desirable to avoid charge trapping for a ferroelectric non-volatile memory device. Other negative impacts of trapping are increased leakage current and earlier breakdown of the ferroelectric/interfacial layer causing a reduced lifetime of the ferroelectric transistor or capacitor. In order to do this, the ferroelectric properties of the ferroelectric material must be enhanced to improve the lifetime of the ferroelectric device.
However, even with improvements to the ferroelectric properties, charge trapping within the ferroelectric layer cannot be avoided completely. For example, due to the ability to make HfO2 thin together with a very thin interface layer while still maintaining its ferroelectric properties (low dead layer effect), charge trapping becomes much more critical compared to other ferroelectric materials such as Lead Zirconate Titanate (PZT), Bismuth Titanate (BTO) or Strontium Bismuth Tantalate (SBT). For PZT, BTO or SBT materials, a layer thickness must be about 100 nm combined with a thick interface layer used as barrier which reduces charge trapping in the ferroelectric material or leakages currents significantly.
To improve the sensing and memory window of the device with thin ferroelectric materials, electrical trapping can be avoided by a reduction of defect sites mainly at the interface to the electrodes. It is known that epitaxial growth of layers is a possibility to reduce defect sites at the interface between ferroelectric materials and the electrode. The reduction of defect sites at the interface needs a nearly perfect match of lattice constant between the different materials. Epitaxial growth therefore limits the available materials significantly because a nearly perfect lattice match of the electrode and dielectric lattice is necessary, which limits the used material sets. Additionally, epitaxial growth is a slow and very expensive approach.